The present invention relates to pre-mixing injection system for supplying a fuel/air mixture to a combustion zone of a turbojet engine combustion chamber.
Conventional present day aircraft turbojet engines have dual annular arrays of combustion heads or modules, typically one array of combustion heads being utilized for low-power operations and the other combustion head array utilized for full-power operations, such as take offs. The combustion heads typically comprise two annular arrays, one located radially outwardly of the other relative to the center line of the combustion chamber. The combustion head array utilized for full-power or take off operations may be located radially inwardly of the other combustion head, or may be located radially outwardly of the other combustion head. Typically, in such dual array combustion head construction, the full-power or take off combustion head is fitted with a pre-mixture injection system comprising an aeromechanical fuel injector accompanied by radial or axial air swirlers located up at the upstream end of a pre-mixing tube.
In such known devices, the pre-mixing tube may assume either a converging or diverging configuration in a direction from the combustion head to the combustion zone of the combustion chamber. A converging pre-mixing tube is used to uniformly accelerate the flow of the air/fuel mixture to assure a safety margin to prevent flashback of the flame into the pre-mixing tube. With this construction, flame stabilization suffers and may be possible only adjacent to the corners of the combustion chamber end. If the pre-mixing tube assumes a diverging configuration, a central recirculation zone within the combustion zone of the combustion chamber is achieved. However, such a construction may allow the flame front to flash back into the pre-mixing tube. In regards to the converging pre-mixing tube, considering the high speeds of the mixture and the leanness of the mixture, the equivalence ratio being approximately being 0.6, the recirculating zones in the edges of the combustion chamber end and the local richness of the mixture make it difficult to achieve satisfactory combustion stability.
European Patent Application 0441 542 discloses a combustion chamber structure having a venturi tube inserted between an annular pre-mixing chamber and the combustion zone of the combustion chamber. This burner is specifically designed for natural gas or hydrocarbon vapors.
French Patent 2,116,363 also discloses the concept of using a venturi tube located between the fuel injector and the combustion zone of the combustion chamber. In this design the venturi tube encloses the conical fuel jet emerging from the fuel injector, the converging portion of the venturi tube accelerating the air around the fuel injector. The divergent portion of the venturi has a wall substantially tangential to the fuel cone. No pre-mixing upstream of the venturi tube takes place in this structure. The purpose of the venturi tube in this device is to achieve adequate mixture richness in relation to stoichiometry in the vicinity of the venturi-tube constriction to prevent combustion in the tube which is also precluded by the speed at which the fuel/air mixture flows.